Question

Synthetic polymers are classified by their sequence of synthesis as chain growth or step growth polymers. Chain growth polymers are produced by chain reaction polymerization in which an initiator adds to a carbon-carbon double bond of a vinyl monomer to yield a reactive intermediate. The intermediate reacts with a second monomer to yield a new intermediate which reacts with a third unit and so on. The initiator may be a radical or an acid or an anion. Acid catalysed polymerization is effective in case of vinyl monomers having electron donating groups. Anionic polymerization requires the presence of electron withdrawing groups. One of the important catalysts used in polymerization is Ziegler-Natta catalyst (triethyl aluminium and titanium tetrachloride). One of the important aspects of Zeiglar-Natta catalysed polymers is that they are stereo-chemically controllable. Three forms (i) isotactic (ii) syndiotactic and (iii) atactic can be produced Vinylidene chloride \(\mathrm{H}_{2} \mathrm{C}=\mathrm{CCl}_{2}\) does not polymerize in isotactic, syndiotactic or atactic forms because (a) it contains electronegative chlorine atoms. (b) it has sp \(^{2}\) hybridized carbon. (c) it is a planar molecule. (d) the polymer formed has no chiral centre

Short answer

Answer: The polymer formed has no chiral centre.

Step-by-step solution

Understand the three forms of polymer structures - isotactic, syndiotactic, and atactic

Isotactic polymers have all their substituents on the same side of the polymer backbone. Syndiotactic polymers have their substituents alternating sides along the backbone. Atactic polymers have a random arrangement of substituents along the backbone.

Analyze the structure of Vinylidene Chloride (H2C=CCl2)

The molecular formula for Vinylidene chloride is given as H2C=CCl2. The carbon atoms are sp2 hybridized, which makes the molecule planar. The molecule has two electronegative chlorine atoms attached to one carbon atom, and two hydrogen atoms attached to another carbon atom.

Evaluate the given options

Option (a): It contains electronegative chlorine atoms. - While it is true that the molecule contains electronegative chlorine atoms, this does not necessarily preclude it from forming isotactic, syndiotactic, or atactic polymers. Option (b): It has sp2 hybridized carbon. - The sp2 hybridization does impact the way vinylidene chloride is polymerized, but it does not explain why it can't form isotactic, syndiotactic, or atactic polymers. Option (c): It is a planar molecule. - The molecule being planar does not directly prevent it from forming isotactic, syndiotactic, or atactic polymers. Option (d): The polymer formed has no chiral centre. - To have isotactic, syndiotactic, or atactic polymers, we need chiral centers on the carbon atoms of the monomer. In the case of vinylidene chloride, both carbon atoms have two identical groups attached (hydrogen or chlorine), meaning that they aren't chiral centers. This explains why it does not polymerize in isotactic, syndiotactic, or atactic forms.

Choose the correct answer

Based on the analysis in the previous steps, we can conclude that the correct answer is (d) the polymer formed has no chiral centre.

Key concepts

Chain Growth Polymers

Chain growth polymers are fascinating materials formed through a process that resembles a chain reaction. This type of polymerization is initiated by a reactive species, such as a radical, an anion, or an acid. These initiators attack a carbon-carbon double bond in a monomer unit, creating an active site that can react with additional monomer units.

As new monomer units continuously add to the growing chain at these active sites, the polymer chain extends rapidly, leading to high molecular weight polymers. One notable aspect of chain growth polymerization is its reliance on specific conditions. For example, acid catalysts work well with vinyl monomers featuring electron-donating groups, while anionic polymerizations often need monomers with electron-withdrawing groups.

Chain growth polymers offer versatility in numerous applications due to their structural properties and the ability to modify them by varying the types of monomers used in the polymerization process.

Step Growth Polymers

Unlike chain growth polymers, step growth polymers grow from the stepwise attachment of monomer units. This process doesn't require an initiator but relies on functional groups on monomers reacting to form bonds, which eventually create long-chain polymers.

The growth between monomer units is slow and uniform, and each step involves many reactions between small molecules. Step growth polymerization often involves condensation reactions where a small molecule, such as water, is eliminated during the polymerization process.

Due to the gradual linking of monomer units, step growth polymerization typically results in polymers with lower molecular weight compared to those formed by chain growth mechanisms. This method is widely used in the production of synthetic polymers like polyesters and polyamides.

Ziegler-Natta Catalyst

The Ziegler-Natta catalyst is a significant breakthrough in the world of polymer chemistry. It primarily consists of a combination of triethylaluminium and titanium tetrachloride.

This catalyst is instrumental in the polymerization of alpha-olefins, allowing for the precise control of the stereochemistry of the polymer. This means that Ziegler-Natta catalysts can dictate how the monomer units are arranged in the three-dimensional structure of the polymer, leading to isotactic, syndiotactic, or atactic configurations.

The catalyst operates under mild conditions and offers not only control over the polymer tacticity but also a high degree of polymerization, yielding polymers that are valuable in automotive, packaging, and many industrial applications.

Isotactic, Syndiotactic, Atactic Polymers

The tacticity of polymers determines the spatial arrangement of the substituent groups along the polymer chain. This is crucial because it influences the physical properties of the resulting polymer.

- **Isotactic polymers** have all substituents on the same side of the polymer backbone. This uniform arrangement typically leads to crystalline structures, resulting in materials that are strong and rigid.

- **Syndiotactic polymers** feature an alternating pattern of substituents on either side of the polymer backbone. This arrangement can lead to semi-crystalline structural properties, balancing flexibility and strength.

- **Atactic polymers** have a random arrangement, leading to amorphous structures, often making them softer and more tacky.

Tacticity is essential as it affects the polymer's melting point, strength, and solubility, dictating its suitability for various applications.

Chiral Centers

Chiral centers in a molecule are carbon atoms bonded to four different groups, leading to non-superimposable mirror images called enantiomers.

In polymer chemistry, the presence of chiral centers is significant for stereo-chemical constructions, such as isotactic and syndiotactic polymers. These isotactic and syndiotactic configurations require chiral centers for the spatial arrangement of the polymer's substituents.

Understanding chiral centers is essential, as they can greatly influence the polymer’s chemical and physical properties, determining the possibility of forming polymers like those catalyzed by Ziegler-Natta catalysts. In the case of polymers like those formed from vinylidene chloride, the absence of chiral centers due to the presence of identical groups on the carbon atoms explains the lack of stereo-chemically controlled forms, such as isotactic, syndiotactic, or atactic polymers.